Haskell：错误：无法将类型“ []”与“ IO”匹配

Question

I'm writing a simple Haskell program that adds an exclamation mark ! to the end of an inputted string by the user. However my program won't run. This is my code:

addExFunction :: String -> String
addExFunction x = x ++ "!"

main = do
 putStrLn "enter string: "
 input <- getLine
 addExFunction input

This is what my error looks like:

a.hs:7:2: error:
    • Couldn't match type ‘[]’ with ‘IO’
      Expected type: IO Char
        Actual type: String
    • In a stmt of a 'do' block: addExFunction input
      In the expression:
        do putStrLn "enter string: "
           input <- getLine
           addExFunction input
      In an equation for ‘main’:
          main
            = do putStrLn "enter string: "
                 input <- getLine
                 addExFunction input
  |
7 |  addExFunction input
  |  ^^^^^^^^^^^^^^^^^^^

I am new to Haskell. How do I fix this error? Thank you.

Answer 1

addExFunction input is a String . In the main do block, you need to use IO actions instead. What do you want to do with the string? If you want to print it, say so:

main = do
   ...
   putStrLn (addExFunction input)